If the car has the remains of the mercedes mobile phone system the FIRST thing i would do is to at least unplug the modules that are involved unless of course you are still using this obsolete sys..the modules are located in the trunk area . The Voice Command sys is part of the phone and is located in the trunk area as well. the orange fiber optic cable need rearranged so the ring remains unbroken..2 cables involved simply a supply and return. isolate these two coming from the front of the car ,cycle the radio power and only one will flash during boot up (obviously the supply) for a 5 secs or so..its not hard if approached inna basic way..otherwise u have no sound..the units were famous for draining the battery with key out..i would aslo try disconnecting the battery for 30 secs or so sometimes that fixes parasitic draws at least temporarily